Internal combustion engines produce large amounts of exhaust gases consisting primarily of carbon dioxide (CO2), water, unburned hydrocarbons (HCs), carbon monoxide (CO) and oxides of nitrogen (NOX). Since the 1970's the emission of unburned HCs, CO and NOX has been regulated and the world-wide regulatory climate for reducing exhaust gases has become ever more stringent. For example, the Clean Air Act Amendment of 1990 mandates that emission generating industrial plants develop and/or implement techniques to significantly reduce their emissions of NOX. Such legislation affects power plants, iron and steel plants, pulp and paper mills, acid production plants, petroleum refineries, lime plants, fuel conversion plants, glass fiber processing plants, charcoal production plants, cement plants, copper smelters, coal cleaning plants, etc. NOX are primary contributors to photochemical smog and acid rain, and may deplete the ozone layer.
As a result, many present day engines, especially gasoline-fueled engines used for passenger automobiles and the like, operate very near stoichiometric conditions, where catalyst technology that allows simultaneous abatement of unburned HCs, CO, and NOX, is well advanced. There is a desire to introduce diesels and gasoline lean-burn vehicles on a broader basis because of their significant fuel economy advantages with attendant lower fuel costs. These vehicles operate with a ratio of air to fuel in the combustion mixture supplied to the engine that is maintained above the stoichiometric ratio so that the resulting exhaust gases are “lean,” i.e., the exhaust gases are relatively high in oxygen content and relatively low in reductants content, e.g., HC, CO, and/or hydrogen (H).
Although lean burn engines provide enhanced fuel economy, they have the disadvantage that conventional three-way catalysts (TWC) cannot adequately abate the NOX component of the pollutants in the gas stream. A number of approaches have been investigated in an attempt to solve this problem. One approach disclosed in JP 8281116 A2 to Shinji et al., is the use of zirconium phosphate in combination with at least one type of NOX storing material. However, in this approach NOX is stored and not chemically altered to NO2 or N2.
In another approach numerous research agencies have investigated the use of non-thermal plasma devices (“NTPDs”) in processes that reduce NOX in gas streams. These techniques use exogenous reducing agents, such as ammonia (NH3), methane (CH4), or carbon monoxide (CO), or neutralizing agents, such as calcium hydroxide (Ca(OH)2). The techniques have utility; however, they are accomplished with relatively low efficiency levels.
In a third approach a two-stage process for reducing NOX emission has been investigated. The first stage plasma converts NOX to NO2 in the presence of oxygen and a catalyst. In the second stage NO2 in the presence of HC and/or carbon soot particles will be converted to N2 and CO2, as disclosed in U.S. Pat. No. 6,038,854. However, in this approach the efficacy of the NOX to NO2 conversion is low.
Considerable research is currently underway toward the development of catalysts that are capable of decomposing or reducing the amount of NOX in emissions under oxidizing conditions. For example, U.S. Pat. No. 6,139,694 to Rogers et al. discloses a process wherein injection of ethanol into a non-thermal plasma reactor in the presence of exhaust gas will significantly enhance the efficiency of the NOX to NO2 oxidation.
Much of this research focuses on using hydrocarbons in engine exhaust to reduce oxides of nitrogen under lean conditions. Interestingly, reacting NO with HC will result in limited N2 production, however reacting NO2 with HC will result in 70-80% production of N2 as a result the NOX/NO2 emissions are decreased.